qemu: domain: Split out setup of virStorageSource from qemu driver config

qemu: domain: Split out setup of virStorageSource from qemu driver config

qemuDomainPrepareDiskSourceData historically prepared everything but
we've split out the majority of the functionality so that it sets up
predominately only according to the configuration of the disk. There
was one leftover bit of setting the gluster debug level from the config.

Split this out into a separate function so that
qemuDomainPrepareDiskSourceData only prepares based on the disk.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
ACKed-by: Eric Blake <eblake@redhat.com>